A water-repellent film, which can repel water and oil and allow easy removal of materials attached to the surface thereof, has been used widely in various fields. For example, by forming a water-repellent film on windows of an automobile, it is possible to secure an excellent view because the windows can repel water even on rainy days.
Furthermore, by forming a water-repellent film on such places as the surface of cooking equipment, kitchen, bathroom, and the like, dirt can be removed easily from such places, and consequently, the care thereof becomes easy. Furthermore, in recent years, such a water-repellent film has been used as a main component of an ink jet head of an ink jet type recording apparatus.
Conventionally, in order to form a water-repellent film on a solid substrate, in general, polytetrafluoroethylene (PTFE) and the derivatives thereof, which have water repellency, have been applied to the substrate to form a film. However, PTFE and the derivatives thereof have a small surface energy and even if they are applied directly to the substrate to form a film, the film peels off from the substrate easily. Therefore, in order to secure the adhesiveness between the film and the substrate, there have been employed a method of roughening the surface of the substrate and then applying a water-repellent film to the roughened surface, and a method of roughening the surface of the substrate, and forming a primer layer (adhesive layer) made of polyethylene sulphide, etc., on the roughened surface, followed by sintering a water-repellent film. Furthermore, when the solid substrate is made of a metal, a method of plating particles of PTFE and the derivatives thereof together with the metal may be employed.
On the other hand, there have been proposed methods of forming a water-repellent film having an excellent adhesiveness directly on the surface of a substrate by using a silane coupling agent without roughening the surface of the substrate. The following are explanations of five conventional examples of methods using a silane coupling agent.
[First Method Example]
There is described a method for forming a water-repellent monomolecular or polymer film by allowing fluoroalkyl trichlorosilane such as CF3(CF2)8C2H4SiCl3 to react with a substrate (for example, publications of JP 2500816, JP 2525536). In the above-mentioned chemical formula, CF3(CF2)8C2H4— represents a fluoroalkyl group, and —SiCl3 represents a trichlorosilyl group. In this method, a substrate having active hydrogen on the surface thereof is brought into contact with a solution in which fluoroalkyl trichlorosilane is dissolved so as to allow a chlorosilyl group (—SiCl) to react with active hydrogen, thus forming —Si—O— bonding onto the surface of the substrate. As a result, a fluoroalkyl chain is fixed to the substrate via the —Si—O— bonding. Herein, the fluoroalkyl chain provides a film with water repellency. Depending on the film formation conditions, the water-repellent film becomes a monomolecular film or a polymer film.
[Second Method Example]
There is described a method in which a porous substrate impregnated with a compound containing a fluoroalkyl chain of fluoroalkyl alkoxysilane such as CF3(CF2)8C2H4Si(OCH3)3 is heated in a vacuum to evaporate the compound, thus providing the surface of the substrate with water repellency (see JP 6 (1994)-143586A). In order to improve the adhesiveness between the water-repellent film and the substrate, this method proposes that an intermediate layer made of silicon dioxide, etc. be provided.
[Third Method Example]
There is described a method of forming titanium, or titanium oxide, indium-tin oxide film on the substrate, and forming fluoroalkyl silane thereon by a vacuum evaporation method (see JP 10 (1998)-323979A).
[Fourth Method Example]
There is described a method of forming fine particles of oxides such as zirconia, alumina, and the like, on the surface of a substrate, and then applying fluoroalkylchlorosilane, fluoroalkylalkoxysilane, or the like (see JP 6 (1994)-171094A).
[Fifth Method Example]
There is described a method of subjecting a mixed solution obtained by adding metal alkoxide to fluoroalkylalkoxysilane, then applying the solution to the substrate and sintering thereof, thereby forming a water-repellent film in which molecules having a fluoroalkyl chain are found in the metal oxide (Publications of JP 2687060, JP 2874391, JP 2729714, JP 2555797). In these methods, a fluoroalkyl chain provides the film with water repellency, and metal oxide provides the film with a high mechanical strength.
Since a water-repellent film using a silane coupling agent can be formed on various substrates without performing a pretreatment, it can be expected to be applied in various fields. It is particularly useful in an ink jet head. However, a conventional water-repellent film using a silane coupling agent lacks durability against alkaline agents.
The conventional monomolecular film or polymer film using a silane coupling agent as mentioned in the above method examples 1 and 2, is bonded to the substrate via —Si—O— bonding. However, since this bonding is hydrolyzed easily in an alkaline solution, when it is dipped in an alkaline solution, it disappears from the substrate. That is, such a film lacks durability in an alkali solution. In particular, in the method mentioned in the second method example, since the adhesive layer is made of silicon dioxide that easily is dissolved in an alkaline solution, therefore this water-repellent film lacks durability in an alkaline solution.
Then, the third and fourth method examples provide a method in which in order to improve the alkali resistance, an alkali resistant lower film made of titanium oxide, titanium, zirconia particles, alumina particles, etc. is formed under a water-repellent film. Thus, a water-repellent film hardly is peeled off from the solid substrate due to the lower layer is breaking away. On the other hand, the problem that hydrogen bonding or siloxane bonding is broken by alkaline has not been solved completely. The reason therefore will be mentioned below. The water-repellent films proposed in the conventional methods use a silane coupling agent having a reactive group only on one end of the linear chain molecule, for example, fluoroalkyl alkoxysilane and fluoroalkyl chlorosilane, etc. In such coupling agents, as shown in FIG. 3, due to the steric hindrance between molecules, three-dimensional polymerization between molecules hardly occurs and the film density is lower than that of a general polymeric polymer. A silane coupling agent 61 causes a hydration reaction with a hydroxyl group on the surface of the substrate to form siloxane bonding, or is fixed by hydrogen bonding. An arrow 62 shows a portion in which the polymerization reaction occurs due to the hydration.
Therefore, as shown in FIG. 4A, as the substrate 71 has the higher density of hydroxyl groups on the surface of the substrate, the density of the film (a film of the silane coupling agent bonded to the substrate) 72 in the vicinity of the substrate becomes higher. Herein, as shown in FIG. 4B, since a lower film 73 is made of titanium oxide, titanium, zirconia, etc., the density of hydroxyl groups on such a film is low, and the density of the water-repellent film (a film of the silane coupling agent bonded to the substrate) 74 that is in contact with the lower film is low.
FIG. 5 is a schematic view showing a state in which water-repellent films 82 and 81 formed on a lower layer 83 having a low density of hydroxyl groups are exposed to an alkali component. In the nearer part of the lower film 83, the silane coupling molecules (a water-repellent film in the vicinity of the lower film) 82 are fixed to the lower film 83 via hydrogen bonding and siloxane bonding, and in the more distant part from the lower film 83, the low density water-repellent film (a water-repellent film distant from the lower film) 81 is formed. When an alkaline ink is brought into contact with this film, ions (OH−) 84 as alkaline components pass through the film 81 and penetrate into the lower film 83. When the density of the water-repellent film 82 in the vicinity of the lower film is small, ions 85 enter the interface between the film 82 and the lower film 83 and break the hydrogen bonding and the siloxane bonding therein. Even if the lower film has much durability against alkali solution, if the density of hydroxyl groups on the surface thereof is low, the alkali resistance of the water-repellent film decreases.
Furthermore, in order to improve the alkali resistance, the fifth method example is useful, in which a molecule having a fluoroalkyl chain is contained in the metal oxide such as titanium oxide, zirconium oxide, or the like, which has the durability in an alkaline solution. However, these metal oxides are required to be produced by subjecting titanalkoxide and zirconiumalkoxide to hydrolysis and dehydration polymerization, and these alkoxides have high reactivity and hydrolysis proceeds quickly in the air. Therefore, a so-called pot life of these alkoxides is short and it is hard to handle a coating solution using these alkoxides for applying a water-repellent film. Therefore, silicone alkoxide that is stable in the air has been used widely. However, silicon oxide formed from the silicon alkoxide is solved in an alkali solution. Therefore, the water-repellent film using silicone alkoxide has a low durability in an alkaline solution.
These water-repellent films produced using a silane coupling agent have a problem that, in general, a silane coupling agent is expensive and the cost of industrial productivity accordingly becomes higher, in addition to the problem that the water-repellent film has a low alkali resistance.